This invention relates generally to closures used to close an opening in a tubular member carrying a gas or liquid under pressure. More particularly, it relates to semi-permanent closures used in connection with hot tapping operations.
In hot tapping operations, it is frequently necessary to close an opening in a tubular member either permanently or semi-permanently, that is, to close an opening where a valve is not required or is not desirable. One example of a semi-permanent closure arises when a hot tap is made into a pipeline or a vessel through a fitting connected to the pipeline or vessel. In the typical hot tapping application as utilized in the petroleum industry, a fitting is welded on the exterior of a pipe that has flowing gas or liquid under pressure. The fitting includes a flange on its outlet connection and a hot tapping machine is secured to the flange. By use of highly specialized equipment, a hole can then be drilled through the wall of the pipe while a gas or liquid continues to flow through it to provide access to the interior of the pipe. This access hole or opening can be used to insert equipment which temporarily blocks flow through the pipe while repairs are being made to the pipe. After the repairs are complete, the equipment is removed but the opening that provides communication to the interior of the pipe needs to be closed. Preferably the closure is made in such a way that at some future date access can be again obtained through the fitting to the interior of the pipe.
Because the tubular member is under pressure, a closure member must be in a properly locked position before the closure member is exposed to atmospheric pressure. An improperly locked closure member can blow out, causing serious injury to an operator or other persons nearby. Therefore, it is critically important that the operator know as a certainty that the closure member is properly aligned and locked before exposing the area above the closure member to atmospheric pressure, and will remain in this aligned and locked position over time. However, because of the challenges associated with completion plug systems above 20 inches and capable of withstanding fully rated 600 lb pressures, while having no leak paths except through the gasket and remaining retrievable, have not yet been developed. Additionally, the ability to set a plug of this size and class without manually removing the holder from the plug has never been implemented. The instability of the plug (due to limited length of engagement between the holder and plug) and the limited flow through the bypass of that system (because the bypass vents the fluid through mating threads) prevented a machine-actuated holder from being used for larger sizes and higher pressures. Finally, anti-rotation has not been implemented for completion plugs of this size or class, as well as smaller sizes and classes. Prior art removable closure systems rely on friction between the o-ring and flange to prevent the plugs from rotating. There is an initial cost savings by not having anti-rotation, but there is more long-term cost savings by preventing pig collisions with the plug or coupon and having the added safety of anti-rotation during the plug setting process.